Momentum operator

This is not really a homework problem but rather a homework-related question.. When I came across my homework (and my textbook: Atkin's physical chemistry 9th Ed.), they defined the momentum operator as:

p_x = - ( [itex]\hbar[/itex] / i ) * d/dx...

but i have seen in other sources that they define it as p = ( [itex]\hbar[/itex] * i ) * d/dx...

they multiply i rather than divide and it is also positive.. i dont known if they represent the same or not, if so.. how can you convert one to the other? Thanks and sorry if this is a very silly question..